Amedeo Avogadro, lithograph, 1856. Credit: The Granger Collection, New York

Two hundred years ago today, a paper proposing an elegant idea—that equal volumes of different gases, at the same temperature and pressure, contain an equal number of molecules—was published in the Journal de Physique. The paper’s author was Italian mathematical physicist Amedeo Avogadro, and his idea became known as Avogadro’s law, which is now a fundamental concept in the physical sciences.

Avogadro’s idea, however, was not initially accepted. In fact, it wasn’t until about four decades later, in the 1850s, that chemists realized the significance of the idea and started working in earnest to make theoretical chemistry consistent with it. One of the leading proponents of this reform movement was Italian chemist Stanislao Cannizzaro, who argued that Avogadro’s hypothesis agreed with the physical and chemical laws discovered up to that time. Shortly thereafter, Avogadro’s idea was adopted by most leading chemists.

To demonstrate Avogadro’s law, one needs to know the number of molecules in a substance. Enter Avogadro’s number (Avogadro’s constant), 6.02214179 × 1023—the number of units in one mole of a substance. The moles of chemistry—far from cute and fuzzy and certainly not conveying the kind of hilarity found in the mole scene in Austin Powers in Goldmember (2002)—are standard scientific units. Yes, they seem quite mundane, and yet, the mole allows scientists to measure vast quantities of entities like atoms, which is simply amazing.

Avogadro’s number provides a startling glimpse into the tiny size of atoms. For example, one gram-mole of water has a mass of 18 grams, equivalent to about 0.63 ounces and containing 6.02214179 × 1023 molecules. The same number of sand grains would be found in a 50-foot-deep layer of fine sand covering the entire state of Texas (the diameter of each sand grain measuring 1/100 of an inch).